Singapore Airlines (SIA), which has been criticised for flying over Ukraine before Thursday's Malaysia Airlines crash, has confirmed that it had been avoiding parts of the country's airspace even before the incident.

It is this same area that the United States Federal Aviation Administration had banned its carriers from flying over in April.

But SIA had flown over other parts of the war-torn country, including the area where flight MH17 went down, where there were no restrictions against commercial flights, the carrier said.

Still, it also said it voluntarily avoids flying over some other parts of the world, even though the areas are open to its aircraft. It did not specify these areas.

Replying to readers who wrote to The Straits Times' Forum Page to raise concerns over SIA flights that used to cut across Ukrainian skies, Captain Gerard Yeap, SIA's senior vice-president for flight operations, said decisions on the use of airspace are made based on numerous factors.

These include weather, safety and security conditions, advisories from international and regional bodies, and any restrictions that may be imposed by the national authorities responsible for the safety of the airspace, he said.

He added that owing to the geographical position of Singapore, SIA had been crossing Ukraine for some of its flights to and from Europe. Even as it did so, "there are several other parts of the world that we proactively avoid flying through, even though they are available for us", he said.

"This has long been our practice, and is the result of our continuous assessment of advisories from the national authorities and aviation bodies," he added.

SIA, along with many other airlines, has been criticised for flying over Ukraine when some other carriers have been avoiding the country entirely.

Still, with the exception of a few carriers - including Australia's Qantas and carriers from South Korea, which all did not operate any flights over Ukraine before the crash - many other airlines such as Germany's Lufthansa and Air India avoided only the closed airspace.

For those airlines that did not use Ukrainian airspace at all, it is possible they did not need to do so when flying the most direct route, aviation experts said. For instance, a flight from London to Dubai would take another route.

Since the crash of MH17 - apparently shot down while crossing a conflict zone in eastern Ukraine where pro-Russian rebels are fighting government forces - commercial carriers, including SIA, have re-routed their flights to avoid Ukraine altogether.

The International Air Transport Association (IATA), which represents global carriers, said yesterday that airlines depend on governments and air navigation service providers to inform them of the routes they can fly, and with what restrictions.

IATA's chief executive and director Tony Tyler said: "Airlines depend on governments and air traffic control authorities to advise which airspace is available for flight, and they plan within those limits. It is very similar to driving a car. If the road is open, you assume that it is safe. If it's closed you find an alternative route."